Molded polyethylene materials find wide usage, for example in gas pipe or water pipe applications. Such materials must have a sufficient stiffness to provide long term dimensional stability. Furthermore, they must have a high impact strength to minimize failures during usage. Advantageously, the materials should also be resistant to crack propagation, that means that an accidental crack or failure should stop and not run over extended length of the material.
It is known to blend polyethylene with various polymers to improve its impact properties. E. Ceaucescu et al. (Revue Roumaine de Chimie, 34, 3, 711-724 (1989)) disclose that the shock resistance of a high density polyethylene (HDPE) can be improved by incorporating a butadiene-styrene block copolymer therein. J. Rhee and B. Crist (Journal of Polymer Science: Part B: Polymer Physics, Vol 32, 159-169 (1994)) discuss the plain strain fracture toughness of blends of HDPE and hydrogenated polybutadiene. W. S. Smith (Soc. Plast. Eng., Tech. Pap. (1975), 21, pages 394-396 describe the use of ethylene-propylene rubber for improving the impact strength and/or environmental stress crack resistance of polypropylene. Alternatively, the ethylene-propylene rubber may be blended with low density polyethylene or high density polyethylene.
Matsuura et al., U.S. Pat. No. 4,330,639 disclose a polymer composition to form polyethylene films which are said to have a high impact strength and a high tear strength. The polymer composition comprises an ethylene/C.sub.3 -C.sub.8 .alpha.-olefin copolymer having an intrinsic viscosity of 1.3 to 8.3 dl/g and a density of 0.850 to 0.930 and an ethylene polymer having a melt index of 0.01 to 0.2, a flow parameter of 1.9 to 2.8 and a density not lower than 0.940.
Bailey et al. (U.S. Pat. No. 4,461,873) disclose ethylene polymer mixtures having a density in the range of 0.940 to 0.965 g/cc and comprising (a) 40 to 70 weight parts of a high molecular weight, low density ethylene polymer having a high load melt index in the range of about 0.1 to about 1.5 g/10 min., a density of about 0.930 to 0.945 g/cc, a heterogeneity index of less than 10 and having essentially no other branching than short chain branching, and (b) 60 to 30 weight parts of a low molecular weight, high density ethylene polymer having a melt index in the range of 45 to 300 g/10 min., a density above 0.950, a heterogeneity index of less than 6 and being essentially linear. The polymer composition is said to be useful for the production of polyolefin films, pipe production and wire coating. The films are said to have good environmental stress crack behavior.
Shirodkar (U.S. Pat. No. 5,041,501) discloses polymer blends of linear low density polyethylene (LLDPE) with a minor amount of an isotactic, partially crystalline butene-1 polymer for producing blown films having improved impact properties. Shirodkar (U.S. Pat. No. 5,242,922) discloses polymer blends of HDPE with a minor amount of an isotactic, partially crystalline butene-1 polymer for producing containers with lower swell compared to those of the unblended HDPE.
Thiersault et al. (U.S. Pat. No. 4,786,688) discloses polyethylene compositions containing from 50 to 98 weight percent of HDPE and from 2 to 50 weight percent of LLDPE with a density below 0.930. The composition is said to be particularly adapted for the manufacture of films and hollow bodies by blow-molding.
Calabro et al. (U.S. Pat. No. 5,102,955) disclose a low density polymer having a broad molecular weight distribution, such that its melt flow ratio is about 50 to about 250, and a substantially constant melt index-corrected density. The polymer is produced by blending a first polymer component of high molecular weight with a second polymer component of low molecular weight, with both polymer components having substantially the same melt index-corrected density. The polymer is said to produce films having improved strength properties, as compared to films made from individual polymer components.
European patent application EP-A-0,517,222 discloses a polyethylene moulding material which consists of from 50 to 80 weight percent of a HDPE, having a density of from 0.940 to 0.960 g/cm.sup.3 and a MFI 190/2.16 of from 0.01 to 0.5 g/10 min. and a broad bimodal molar mass distribution, and of from 20 to 50 weight percent of a polyethylene of a density of from 0.910 to 0.925 g/cm.sup.3 and a MFI 190/2.16 of from 0.5 to 2.0 g/10 min. The polyethylene moulding material has a density of from 0.930 to 0.940 g/cm.sup.3 and a MFI 190/2.16 of from 0.05 to 1.0 g/10 min. The polyethylene of low density may be a linear polyethylene (LLDPE) with a narrow unimodal molar mass distribution. The European patent application discloses that pipes, sheets and films of good long term and low temperature properties can be produced. Particularly, the material is said to have extraordinary resistance to slow crack propagation (ESCR).
Additionally, ternary polymer blends are known. For example, in U.S. Pat. No. 4,824,912, Su et al. disclose LLDPE blended with minor amounts of a low molecular weight HDPE (LMW-HDPE) and a high molecular weight HDPE (HMW-HDPE) for processability and film property improvements over LLDPE used alone. The disclosures of all the above-cited references are incorporated herein by reference.